Hi,
if you are staying in windows, windows xp x64 is of writing very stable, provided you have some decent hardware. x64 is able access and address more RAM then its x32 counterpart.
Even though Adobe PS isn't a x64bit app it is still able access more ram then in windows xp x32 (AFAIK). I am sure Vista x64 will be similar but should be able to access even more memory again (Ultimate version)
However, your system, regardless of OS, is only as good as your weakest link and usually it is limited by your harddisks sub-system. As suggested by others here, a RAID-0 (Stripping) would be faster with 2 or more drives (well, you have to have atleast two otherwise there is nowhere to stripe to/with :-) ) and surely a SolidStateDrive would be faster again, however, if you have money for the later, then you might as well get as much ram as you can.
In order to get more the 4Gb, you will mostlikely have to get a workstation mainboard which supports more then this, there are a couple of desktop single processor boards, that came out last or early this year, that could take 8gb of ram, but with the workstation boards, you can use 16, 32, 64 or even 128Gb or ram (tho its going to cost!) see
www.tyan.com or
www.supermicro.com and ofcourse
www.intel.com however, the first two are usually faster and better featured.
I think the MacPro is maxing out at 16Gb using Intel boards 5xxx series.
if you have any questions, ask away!
Good luck
Henrik
PS: Being a PC person, I have to admit that the Macpro offers very good performance and value for the money (...don't ever quote me of saying this!) :-)